The present invention relates to bulk acoustic wave resonators, such as are used in providing bulk acoustic wave filters. More particularly, the present invention relates to bulk acoustic wave resonators with a particular variety of acoustic mirror, one including only metallic layers.
A thin film bulk acoustical wave (BAW) filter can be fabricated on various substrates, such as silicon (Si), gallium arsenide (GaAs), or silicon dioxide (SiO2). A thin film BAW filter often includes a shunt BAW resonator and a series BAW resonator, although some filters include only a series resonator and a shunt capacitor or only a shunt resonator and a series capacitor, and other resonators are based on topologies such as the lattice filter topology. A BAW filter including a series resonator and a shunt resonator could also include several such pairs of resonators so as to form what is called a ladder filter. Each BAW resonator includes a top electrode deposited atop a layer of piezoelectric material, which in turn sits atop a bottom electrode. The assembly of these three layers is sometimes referred to as the resonator section of a BAW resonator. A BAW resonator further includes other layers of materials, or different structural arrangements of materials, so as to finetune the performance of the BAW resonator.
Two types of BAW resonators are known in the art: a bridge type BAW resonator and an acoustic mirror type BAW resonator. In a bridge type BAW resonator, the resonator section is deposited on a membrane (made from one or more layers of different materials) and acoustic waves generated by the resonator are reflected back from the air interface above the top electrode, and from the air interface below the bottom electrode. In a mirror type of BAW resonator, the resonator is solidly mounted on top of a stack of layers making up what is called an acoustic mirror. The layers are selected to present to acoustic waves created by the resonator section alternately high and low acoustic impedance. Each layer of material in an acoustic mirror is typically one quarter of a wavelength thick. Such an acoustic mirror provides for a large reflection factor back toward the resonator for acoustic waves created by the resonator propagating in the direction of the acoustic mirror. There is also a reflection of the acoustic waves created by the resonator at the air interface at the top electrode.
Both types of BAW resonators have the disadvantage that they require a protective package with an air cavity over the resonator section. The packages typically used are similar to those used for SAW-filters, i.e. hermetic or at least semi-hermetic sealed ceramic packages. Such packages increase the size of the components and also the price. In addition, such packages create parasitic inductances and resistances.
What is needed is a structure that provides the required high-reflection factors both above and below a resonator section, and obviates the need for hermetic or semi-hermetic packaging with its attendant parasitic inductances and resistance and other disadvantages.
Accordingly, the present invention provides a method of fabricating a bulk acoustic wave (BAW) resonator and a BAW resonator so fabricated, the method including the steps of: providing a substrate; providing a first isolation structure; and providing a resonator section, the resonator section comprising a piezolayer; wherein the first isolation structure comprises an acoustic mirror made from only electrically conductive layers of alternating high and low acoustic impedance.
In a further aspect of the invention, a layer of the acoustic mirror abuts the piezolayer and serves as an electrode.
In another, further aspect of the invention, the first isolation structure is situated between the resonator section and the substrate.
In yet another, further aspect of the invention, the first isolation structure is situated above the resonator section, on the side of the resonator section facing away from the substrate, so that the resonator section lies between the first isolation structure and the substrate, and the method further comprises the step of providing a second isolation structure situated between the resonator section and the substrate. In some such applications, the second isolation structure includes an acoustic mirror made from layers of materials of alternating high and low acoustic impedance, and in other such applications, the second isolation structure includes a membrane. In yet still other such applications, the method of the invention also includes the step of providing a flip-chip ball on top of the first isolation structure.
In yet still another, further aspect of the invention, the method also includes the step of providing a capping material, the capping material positioned so that the first isolation structure and resonator section are sandwiched between the substrate and the capping material and so protected against mechanical loading, with an air interface between the resonator section and either the capping material or the substrate in case of the resonator including only a single isolation structure.